Offer two or 3 Waxworms occationally, but note that they are high in fat. Avoid Mealworms and use their larger relative called Superworms. They have a little less chitin in their exoskeleton and are much larger. Only offer these items as treats.
It may take time to get your frog to eat earthworms since it has had a deprived diet. Keep trying though. He may come around.